Middle-income couples, recently identified by the Conservatives as the "struggling middle", are increasingly unable to afford to separate when their relationships end, according to a new study.

Almost half the counsellors surveyed at Relate, the charity that specialises in relationship counselling, say an increasing proportion of the 150,000 clients they see each year are being forced to remain living together despite having decided to split up. Couples with children are more likely to find themselves trapped than those without, but both groups are increasingly finding it impossible to bear the cost of setting up different homes.

"When we talk about Relate's clients, we are not talking about people on low incomes. We're talking about people in employment, on average to above-average incomes," said Ruth Sutherland, the charity's chief executive.

Sutherland said the charity, which began life as the Marriage Guidance Council 75 years ago, had never seen this demographic of clients struggling with their finances to such an extent that moving into two homes and getting on with their lives was an impossibility.

"These are peoplewho could previously afford to move away from each other when their relationship broke down," she added. "But now, they are stretched just to pay their mortgage on top of the rising cost of living. When their relationship breaks down, they find they can't afford two mortgages, on top of the cost of running two homes."

Sutherland said that for parents, the cost of childcare was another devastating factor. Parents in the UK spend an average of 27% of their salary on childcare, compared with a European average of 13%. Twenty-five hours of nursery care a week for a child aged two or under costs on average £5,000 in England, rising to between £6,000 and £15,000 in London.

"To pay for the increased childcare demands that come with being a single parent has become a pipe dream for many people, even those in well-paid jobs," said Sutherland.

Richer couples could find themselves in the same predicament as the difficult economic climate continued, Sutherland predicted. "I would not be surprised at all to see the problem creeping up the salary band," she said. "This era of austerity we're in is not like other hard times we have lived through.

"In the past, we've had a dip and then recovery, but now we're in unknown territory about the length of time people are going to have to cope with debt, job insecurity, pressure from work and the mounting cost of childcare.

"The only thing we know is that people are going to have to cope with these problems for longer than they would ever have done so before."

At least 40% of Relate counsellors said they were seeing more couples split up than two years ago, with money worries cited as a major cause.

"It's vital for the future of our children, and thus the future health of our nation, that estranged parents manage their separation well," said Sutherland.

"Children learn about relationships at home. If they see their parents undermining each other, arguing and being vindictive, then that's the foundation on which they will build their own relationships. It's not only the adults who, if stuck in a toxic situation, are going to be damaged."

Which is why, said Sutherland, she was so concerned by another finding in Relate's survey: that separated couples are increasingly unable to afford to complete their counselling courses.

At least 80% of counsellors said increasing numbers of clients were unable to afford to "properly start or conclude" their counselling programmes, despite being offered short, intensive courses of four to six sessions, charged from £6 to £45 an hour, depending on their income.

Over 70% of Relate counsellors said money problems including debt, a lack of disposable income, unemployment and rising living costs had worsened for their clients in the last two years.

Almost 90% of counsellors said money worries made their clients depressed, with 80% saying couples argued more as a result and 65% saying it affected their clients' physical health.

"Let's all be clear about the real cost of austerity: the impact of being in a relationship that isn't working is toxic. It is harmful to your children and it permeates every other aspect of your life," said Sutherland. "If the government wanted to protect the mental health of the country, both now and in the future, they would target these cuts differently."

Nearly a quarter of people have continued to live with a partner, or know someone who has, because they couldn't afford to live apart, according to a 2010 report from Shelter. "We also know that relationship breakup is a major cause of homelessness," said Campbell Robb, chief executive of Shelter.

The 2012 total cost of family breakdown to the UK was £44bn, up from £42bn in 2011, according to a recent study by the Relationships Foundation. The study looked at the cost of family breakdown in five key areas of public policy: tax and benefits, housing, health and social care, civil and criminal justice, and education and young people not in education, employment or training (Neets). It concluded that the annual cost for each taxpayer was now £1,470.

"The government's austerity policies are making things worse, and it doesn't make sense economically," said Sutherland. "What we want is for them to do a relationship and family impact assessment for every policy they consider introducing."

Robb said the "shortage of affordable housing in this country is being felt further and further up the income scale".

"We're hearing from couples moving in together too fast to help with housing costs but then unable to move out if things go wrong because they can't afford to live on their own. This has a huge impact on people's home lives," he added.

Robb said the housing crisis is "the result of … more and more people chasing fewer and fewer homes, which has pushed up house prices and rents far faster than wages have risen.

"Our research also shows that more and more people are putting off having children because they can't find an affordable home," he said. "Something is badly wrong when people who are working hard still face a constant struggle to get a decent place to live."

Caroline Davey, director of policy at Gingerbread, the charity for single-parent families, said families in the low- to middle-income bracket were "increasingly struggling financially". "When a couple separates this financial squeeze can make it impossible for them to forge new lives separately," she said.

"With wages stagnating, higher risk of redundancy, spiralling living costs, and many families without any savings to speak of, it can be simply unachievable for a separating couple to afford to run two homes rather than one. The only alternative for some families is to continue living in the same home but as separate households."

Davey warned: "This situation could become more commonplace in future as the financial downturn bites even harder on families across the income scale."

She added: "Action is needed across a number of areas, for example strengthening the role of local authorities in supporting access to private rented accommodation, reversing the harshest housing benefit cuts, and sustained job creation."

A spokeswoman for the Treasury said: "The government has taken action to help people with the cost of living, including freezing council tax and fuel duty and cutting income tax for 25 million people by raising the personal allowance. Action taken to reduce the deficit has helped to keep interest rates near record lows. And we have extended the offer of 15 hours free education and care a week for disadvantaged two-year-olds, to cover an extra 130,000 children."

Case study

Adela and Tanek married in Poland in 2002 and came to Britain with their two children four years later. For a few years, they lived comfortably: Adela worked full-time as an administrator, and Tanek in a factory. "Our finances were fine – more than fine," said Adela. "We were living well and saving money. The children were happy and life was good."

Two years ago, however, the couple broke up. Adela moved into a studio flat. The parents shared custody of the children but Tanek had returned to university and, soon afterwards, Adela's office closed down. Money became tight.

"Despite the problems, we would have had enough money but rents were going up and the cost of living rose sky high," said Adela. "We found that we just couldn't run two households, no matter how cheaply we lived. I didn't want to move into a single room because I wanted the children to live with me for half the week.

"It was a difficult decision and a terrible step backwards, but we eventually decided we had no choice but for me to move back into the family home."

They are forced to share a bed but Adela said that, in one way, she and Tanek were fortunate. "We get on well as friends but this situation is terribly awkward and very wrong. We want to get on with our lives and meet new people but we're stuck together.

"We can't afford a second bed and have nowhere to put one anyway. We don't have a sofa we can sleep on and we don't want the children to have to share their room with one of us."

They hope that when Tanek finishes his degree next year, they will be able to afford to live separately. "But who knows?" said Adela. "The way the economic situation is at the moment, he might be unemployed for a long time."

• This article was amended on 21 November 2012. The original said Relate was founded 25 years ago. It began life as the Marriage Guidance Council nearly 75 years ago and was relaunched as Relate in 1988. It also said almost half the 2,000 counsellors at Relate reported that couples were having to stay together because they could not afford to split up. Half the counsellors surveyed said that; 10% of Relate's 2,000 counsellors were surveyed.